(a) Field of the invention:
This invention relates to an automatic focus detecting device for such optical systems as microscopes and cameras and more particularly to an automatic focus detecting device wherein object images are received by a photoelectric converting device consisting of a plurality of elements, an output signal representing the received light amount is obtained from the respective elements and the detection of whether an object image formed is in-focus or not is performed on the basis of these output signals.
(b) Description of the prior art:
Among conventional automatic focus detecting devices of this kind, there are a range finder type wherein a trigonometric measurement is applied and a TTL system wherein a light pencil passing through a pupil is divided to obtain two images. In either of them, correlation of two images is digitally determined to detect which, the coincidence of the two images is indicated with the maximum of the correlative value and the relative movement amount of the two images is indicated by the phase difference amount of the images.
FIG. 1 shows an example of such conventional automatic focus detecting devices. The data A and B of two images picked up by an image sensor not illustrated are memorized respectively in ring-shaped shift registers 1a and 1b through a sample holding circuit, A-D converter (not illustrated either) and others. In this example, the image data are formed of 128 elements. When both image data A and B are addressed respectively in the shift registers 1a and 1b, then the absolute values of the differences of the respective elements will be determined by circuits 2 for determining the absolute values of the differences and further the sum of the absolute values will be determined by an adding machine 3 to obtain the correlative value of two images. Then, the image data B of the shift register 1b will be shifted by 1 element by a pulse from a clock CL and again the correlative value will be determined by the circuit 2 and adding machine 3. Thus, whenever the image data on one hand are shifted in turn by the clock CL, the correlative value will be determined, further the extreme value of the correlative value will be determined by a peak detector 4 and the position in which the extreme value is detected will be an in-focus position. Also, the clock number in the case of the extreme value will be determined by a counter 5, this clock number, that is, the shifting amount of the image data B of the shift register 1b will be the phase difference amount of the two images and the direction and amount of the de-focus will be able to be known from the phase difference amount.
However, in this conventional device, as the image data obtained by the image sensor are used as it is, in case the light amounts of two image data are not uniform due to the eccentricities of an optical system and/or the pupil or in case the light amount distributions of two image data are not uniform due to a difference of position between a pupil divider and the pupil, there are such problems that a wrong focusing will be performed, the focusing becomes impossible or the like. Further, in an automatic focus detecting device of the other type than the two images coincidence type, in which a pair of image sensors is placed before and behind an image forming plane and the balance between the light amounts to be received by both image sensors is detected to focus, there are the same problems as described above.